1. Field of the Invention
The present invention concerns a resonator for magnetic resonance applications of the type having a conductor element extending in an extension direction.
2. Description of the Prior Art
Resonators of the above type are generally known.
Like many other resonators, resonators for magnetic resonance applications have a conductor element that extends in an extension direction. Given operation of the conductor element with a resonance frequency, a resonance current oscillates in the extension direction in the conductor element. The resonance current is particularly high when the conductor element is tuned to the resonance frequency.
In magnetic resonance applications the Larmor frequency with which a particular magnetic resonance system is operated depends on the strength of the basic magnetic field of the magnetic resonance system and on the element whose excited spin should be detected. For example, for hydrogen the gyromagnetic ratio is approximately 42.4 MHz/T.
Magnetic resonance systems are typically operated with basic magnetic fields that are between 0.2 and 1.5 T. In more recent times magnetic resonance systems have become known that exhibit stronger basic magnetic fields, in particular basic magnetic fields of 3 T, in some cases even up to 7 T and beyond. The Larmor frequency of such magnetic resonance systems typically lies between 8.5 MHz and approximately 63.5 MHz, but may be beyond this in individual cases.
The Larmor frequency is the frequency to which the resonators should be tuned in magnetic resonance applications. In the ideal case it thus corresponds with the resonance frequency of the resonator.
As is generally known, without further measures a conductor element is resonant at a resonator frequency when its length is an integer multiple of half of the wavelength of the resonance frequency. As results from a back calculation, the length of a λ/2 rod given a magnetic resonance system with a basic magnetic field of 1.5 T is thus approximately 2.5 m. However, such lengths are unrealistic for resonators for magnetic resonance applications. For example, the rods of whole-body antennas exhibit lengths that are at maximum 60 cm. They thus exhibit (as viewed in their extension direction) a length that is not only smaller than half but rather (at least normally) is even smaller than one quarter of the wavelength of the resonance frequency. Local coils are often even smaller. For resonators for magnetic resonance applications, without further measures it is therefore not possible to achieve the tuning to the Larmor frequency only by dimensioning of the conductor element. Rather, it is generally typical to provide suitable circuitry that effects the tuning of the conductor element to the resonance frequency.
The resonance current is in the radio-frequency range. In the propagation of radio-frequency currents, a phenomenon known as the skin effect occurs, such that the resonance current no longer flows in the entire cross-section of the conductor element but rather only in a peripheral region. The peripheral region exhibits a skin depth that is determined by the resonance frequency and the material of the conductor element. Due to the skin effect, the resonance current thus flows only in a fraction of the cross-section of the conductor element, such that the effective resistance of the conductor element rises.
It is conceivable to reduce the effective resistance of the conductor element by cooling or by the use of a superconducting material. However, these procedures have a significant technical expenditure associated therewith and moreover represent a safety risk for a patient who is examined in the magnetic resonance system. In practice these measures are therefore not ever used in magnetic resonance systems.
Theoretically, the use of a radio-frequency strand is conceivable (see WO 97/26560 A1). However, in practice the use of a radio-frequency strand brings no advantages. Stranded conductors reduce the resistance only up to frequencies of a few megahertz, typically 2 to 4 MHz. However, the Larmor frequency normally lies well above this frequency, namely at at least 8.5 MHz.
Conductor elements are known that are fashioned as multi-layer conductors as exemplified by U.S. Pat. Nos. 2,769,148 and 6,148,221. When in such a case the individual layers exhibit layer thicknesses that are smaller than the skin depth, the effective resistance at the resonance frequency can be significantly reduced with such conductor elements. The layers can either be concentric to one another (known as Clogston conductors as in U.S. Pat. No. 2,769,148) or planar (as in U.S. Pat. No. 6,148,221). If such conductor elements could be used in resonators for magnetic resonance applications, this would be advantageous. However, without further measures the use of multi-layer conductors as conductor elements does not lead to the expected reduction of the effective resistance.
More precise examinations have shown that the problem is due to the optimal distribution of the resonance current to the individual layers of the multi-layer conductor after a transition from a solid conductor or an external circuit to the multi-layer conductor ensuing only when this is ensured by corresponding design of the multi-layer conductor or the external circuit. The current distribution can not be adjusted without further measures. Moreover, slight inhomogeneities of the multi-layer conductor already lead to a significant reduction of the achievable resistance reduction. The use of multi-layer conductors in resonators for magnetic resonance applications has conventionally been considered to be unreasonable in practice.